block: Add bdrv_lock()/unlock()

Inside of coroutine context, we can't directly use aio_context_acquire()
for the AioContext of a block node because we already own the lock of
the current AioContext and we need to avoid double locking to prevent
deadlocks.

This provides helper functions to lock the AioContext of a node only if
it's not the same as the current AioContext.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201005155855.256490-14-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

block.c

@@ -6326,6 +6326,33 @@ void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx)
     bdrv_dec_in_flight(bs);
 }
 
+void coroutine_fn bdrv_co_lock(BlockDriverState *bs)
+{
+    AioContext *ctx = bdrv_get_aio_context(bs);
+
+    /* In the main thread, bs->aio_context won't change concurrently */
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+
+    /*
+     * We're in coroutine context, so we already hold the lock of the main
+     * loop AioContext. Don't lock it twice to avoid deadlocks.
+     */
+    assert(qemu_in_coroutine());
+    if (ctx != qemu_get_aio_context()) {
+        aio_context_acquire(ctx);
+    }
+}
+
+void coroutine_fn bdrv_co_unlock(BlockDriverState *bs)
+{
+    AioContext *ctx = bdrv_get_aio_context(bs);
+
+    assert(qemu_in_coroutine());
+    if (ctx != qemu_get_aio_context()) {
+        aio_context_release(ctx);
+    }
+}
+
 void bdrv_coroutine_enter(BlockDriverState *bs, Coroutine *co)
 {
     aio_co_enter(bdrv_get_aio_context(bs), co);